We have shown that dark pigmentation independent of race or ethnicity, endows the epidermis with enhanced barrier function and antimicrobial defense. As a result, lightly-pigmented skin should be inherently more at risk for development of inflammatory skin disorders, such as atopic dermatitis and adult eczemas, well as all types of skin infections. But the basis for the link between pigmentation and epidermal function remains unknown. Using organotypic human keratinocyte cultures, grown with melanocytes, as well as selected mouse models, we will assess first, the role of melanocyte density and the extent of melanocyte pigmentation in regulating epidermal function. The functional end-points will comprise changes in permeability barrier homeostasis, stratum corneum (SC) integrity/cohesion, and expression of certain key antimicrobial peptides that contribute to cutaneous antimicrobial defense. Since our recent studies suggest that darkly-pigmented melanocytes influence epidermal function by acidifying the outer epidermis, we next will assess the cellular basis for epidermal acidification by melanocytes; and then in mutant mouse models, which step in melanosome formation, acidification, or secretion regulates this process. If acidification alone fails to account for the impact of melanocytes on epidermal function, we plan to search for other melanocyte-initiated regulatory signals that influence epidermal function in a paracrine fashion. In Aim 3 we will examine the converse issue; i.e., how alterations in epidermal barrier requirements, elevations in surface pH, or a reduced environmental humidity ( UV-B) could regulate interfollicular (epidermal ) pigmentation, which then would improve barrier function in a feed forward fashion. Our prior studies have already shown that barrier disruption increases both IL-1a and NGF production, and that dose of UV-B that damage the barrier upregulate p53 expression. These signals are known to either increase epidermal pigmentation by increasing epidermal POMC expression and proteolytic processing to aMSH, and/or MC1R signalling of melanocyte function. We have identified two other keratinocyte regulatory signals could also link changes in epidermal barrier function to pigmentation; i.e., i) epidermal stem cell factor, which induces melanocytes to migrate from follicles to interfollicular epidermis; and ii) the transcription factor, foxn1, which targets melanocytes to the basal layer. We will assess whether altered barrier requirements regulate one or more of these 5 signaling mechanisms, leading to increased expression or activity of the key downstream enzymes of melanin production; i.e., tyrosinase, tyrosinase hydroxylase, and/or melanosome acidification. Finally, we will assess whether TRPV4, the epidermal sensor or external humidity, regulates pigmentary responses to decreased environmental humidity. Together, these studies will provide new insights into the role of pigmentation in regulating epidermal function, potentially explaining differences in disease susceptibility in different pigment groups, and perhaps leading to disease prevention in lightly-pigmented epidermis. Conversely, they should pinpoint epidermal-derived signals that regulate the development of interfollicular pigmentation in response to exogenous stress.